For the treatment of blood-related diseases including leukemia as a typical example, it is extremely important to stably amplify and supply blood cells in an amount necessary for such treatment. Thus, to date, many researchers have attempted to efficiently amplify hematopoietic stem cells or hematopoietic progenitor cells. Among blood cells, megakaryocytes are cells capable of producing proplatelets, and further, platelets. Hence, they occupy an important place in therapeutic applications. Among blood cells, platelets are cells essential for blood coagulation (hemostasis). Accordingly, the demand for platelets is extremely high in leukemia, bone marrow transplantation, anticancer therapy, etc. To date, the platelets have been supplied by a method involving collection from blood donors. However, such method involving collection from blood donors would hardly achieve stable platelet supply due to chronic shortage of donors, inability to preserve the collected platelets in a frozen state, etc. At the same time, attempts have been made to apply a method involving administration of TPO, a method of differentiating megakaryocytes from umbilical cord blood or myelocytes, etc. However, such TPO administration has not yet been put to practical use because antibodies neutralizing the TPO are produced after administration of the TPO. Also, the method of differentiating megakaryocytes from umbilical cord blood or myelocytes is not suitable as a method of stably providing platelets because it is able to obtain only small quantities of hematopoietic stem cells serving as a source of megakaryocytes.
In recent years, in vitro amplification of hematopoietic stem cells or hematopoietic progenitor cells, which are obtained only in a slight amount from living bodies, has actively been conducted. For example, there have been reports on: a method for establishing, from mouse ES cells, a hematopoietic stem cell line capable of self-renewing and differentiating even into lymphocytes (Patent Document 1); a method comprising inducing in vitro ES cells of a primate to differentiate, then transplanting in utero the obtained cells into a sheep fetus, and obtaining differentiated primate hematopoietic cells from the newborn lamb (Patent Document 2); and a method for conveniently and stably amplifying in vitro CD34-positive/CD38-negative cells that maintain undifferentiation of hematopoietic stem cells (Patent Document 3).
In order to stably supply platelets, a method for allowing hematopoietic stem cells or hematopoietic progenitor cells to efficiently differentiate into megakaryocytes and platelets. Therefore, attempts have actively been made also to induce megakaryocytes, and further, platelets, from ES cells derived from various types of animals. Eto et al. have revealed that mouse ES cells are induced to differentiate into megakaryocytes by coculturing them with OP9 stromal cells (Non-Patent Document 1). Fujimoto et al. have reported that platelet induction had been confirmed using a method similar to that of Eto et al. (Non-Patent Document 2). Moreover, there are also the following reports: a report regarding successful induction of megakaryocyte differentiation from monkey ES cells (Non-Patent Document 3); and a report regarding successful induction of megakaryocyte differentiation from human ES cells (Non-Patent Document 4). However, the release of platelets has not confirmed in any of these documents. Moreover, in order to stably obtain a therapeutically necessary amount of blood cells, other than platelets or megakaryocytes, it is necessary to efficiently obtain hematopoietic stem cells or hematopoietic progenitor cells. However, it is hard to say that such method has been established.
Patent Document 1: Japanese Patent Laid-Open No. 2006-141356
Patent Document 2: Japanese Patent Laid-Open No. 2004-350601
Patent Document 3: Japanese Patent Laid-Open No. 2006-61106
Non-Patent Document 1: Eto et al., Proc. Acad. Sci. USA 2002; 99: 12819-12824.
Non-Patent Document 2: Fujimoto et al., Blood 2003; 102: 4044-4051.
Non-Patent Document 3: Hiroyama et al., Exp. Hematol. 2006; 34: 760-769.
Non-Patent Document 4: Gaur et al., J Thromb Haemost. 2005; 4: 436-442.